The invention relates to a method of producing a structural part from sheet metal, in particular a body sheet, and to an embossing tool for producing such a structural part.
Many structural parts in automobile body construction must fulfill stringent requirements with regard to the inherent stability and bending stiffness. Such structural parts are as a rule produced from sheet and are normally provided with beads for increasing the inherent stability and bending stiffness. However, the beading of the sheets leads to a highly directional stiffness increase: thus, although sheet-metal strips having beads arranged in the longitudinal direction are certainly flexurally rigid in the longitudinal direction, they have only low torsional rigidity and are flexible in the transverse direction. Uniform homogeneous stiffening of the sheets therefore cannot be ensured by embossing such beads.
Stiffening with substantially less directional dependence can be achieved if the sheets—instead of being provided with the beading—are provided with “vault structuring” by bulge forming; this method is described, for example, in DE 44 37 986 A1. In contrast to the conventional forming processes (such as deep drawing for example), in which plasticizing of the sheet takes place during the forming, the sheet is merely folded locally during the vault structuring; this is associated with only a small surface enlargement of the material. The bulge forming is effected in a continuous process, so that entire sheets of material (or band-like regions on them) are provided with the vault structure. To produce body structural parts having increased stiffness, blanks are cut out of these sheets of material, and the desired structural parts are then produced from these blanks by further process steps. Thus, for example, DE 199 42 383 A1 describes a motor vehicle floor which is produced by deep drawing a vault-structured sheet.
However, difficulties occur if body parts of vault-structured sheet are to be connected to other structural parts using conventional joining processes (spot welding, screwed connections, etc.). This is because, on account of the vault structuring of the structural part, it cannot be ensured that the members to be joined meet one another in a planar manner in the connection zones in which the vault-structured structural part is to be joined to other structural parts. In order to be able to ensure controlled, high-strength joining of such structural parts by means of a welding process—or a positive-locking screwed connection—the vault structuring of the structural parts must be removed to the greatest possible extent in the connection zones so that the structural parts in these connection zones can bear in a planar manner on (flat) connecting regions of other adjacent subassemblies.
The object of the invention is to propose a method by means of which vault structures can be smoothed locally. It is also the object of the invention to provide a tool which permits simple and controlled smoothing of a locally limited connection zone on a vault-structured structural part.
This object is achieved according to providing a method of producing a structural part from sheet metal, in particular a body sheet having a stiffening zone in which the structural part is provided with vault structuring which increases the stiffness, and having a connecting zone which is more or less free of vault structures, a vault-structured cut-to-size sheet being used as a blank for producing the structural part, said method comprising the following method steps: placing the blank into an embossing tool having a punch and a die; lowering the die with the blank being fixed relative to the punch in a region of the vault-structured stiffening zone by a fixing element which is in a leading position with respect to the die; and further lowering of the die to shape, the connection zone between the punch and die.
This object is also achieved according to the invention by providing an embossing tool for producing a structural part having a vault-structured stiffening zone and a connection zone largely free of vault structures, starting from a blank of vault-structured sheet, the embossing tool having a punch and a die, wherein the die has a leading fixing element for fixing the blank in the tool and also has a basic die body, wherein the punch of the embossing tool is of multi-piece design and comprises a fixing punch interacting with the fixing element of the die, and a forming punch displaceable with respect to the fixing punch, and wherein the basic die body and the forming punch are provided with interacting smoothing regions for embossing the connection zone on the structural part.
Accordingly, in order to produce the desired structural part, a structural-part blank, i.e. a suitably shaped vault-structured sheet-metal section, is formed in an embossing tool comprising a punch and a die. The embossing tool is designed in such a way that the vault structuring originally present on the structural-part blank is not affected in the region of the stiffening zones, whereas the vault structure is smoothed in the connection zones. In order to achieve this, the punch of the embossing tool is of multi-piece design: the punch comprises a fixing punch which is arranged in a region of the punch corresponding to the stiffening zone of the structural part and whose function—in interaction with the leading fixing element on the die—is to fix the blank in the tool without deforming it. Furthermore, the punch comprises a forming punch which is displaceable relative to the fixing punch and is arranged in the region of the punch corresponding to the connection zone of the structural part and whose function—in interaction with a part of the die facing this forming region—is to smooth the sheet locally in the connection zone.
To produce the structural part, first of all, the fixing punch of the embossing tool is raised, and the structural-part blank is put onto the fixing punch in the desired orientation; by lowering the die of the embossing tool, the blank is then clamped in place relative to the punch in the region of the stiffening zone by the leading fixing element of the die; the position of the blank in the tool is therefore fixed. The die is then lowered further, in the course of which the connection zones are formed between the forming regions, corresponding to one another, on punch and die.
Smoothed connection zones in which the vault structure is mostly removed can therefore be produced on the vault-structured blank by means of the embossing tool according to the invention. In these connection zones, the structural parts can be connected to connection parts in following production steps with the aid of conventional joining processes—for example by spot welding or seam welding during the body-in-white stage or by screwed connections during assembly. In the process, the vault structure of the stiffening zones is retained without being deformed. Therefore structural parts which are provided in selected stiffening zones with a (more or less directional) vault structure, but have a more or less smooth surface structure in adjacent connection zones, can be reproduced by means of the embossing tool according to the invention.
In an advantageous configuration of the invention, the end face of the fixing punch is provided with a surface contour which corresponds to the vault structuring of the blank. This has the advantage of helping to orient and position the blank in the tool in a defined manner when the blank is put into the embossing tool, this orientation and position corresponding to the surface structure of the fixing punch. In this way, in particular, the vault structure of the sheet can be oriented in a highly precise manner relative to the forming regions of the embossing tool, so that the smoothed connection zone produced in the course of the embossing operation is positioned in a highly precise manner relative to the vault structure of the stiffening zone.
The fixing element, interacting with the fixing punch, of the die is preferably made of an elastic material, in particular rubber. When the fixing element guided in the die is lowered onto the vault-structured surface of the blank to be fixed, the end face of the fixing punch elastically adapts itself to the vault structure; during the further lowering of the die, the fixing element is compressed, as a result of which the fixing force, exerted by the fixing element, on the blank is increased. The blank is therefore clamped in place with high force (and thus in a nonslip manner) between fixing punch and fixing element during the smoothing of the connection zone.
Alternatively, the fixing element may be designed as a spring-mounted slide, the end face of which is provided with a vault structure (complementary to the end face of the fixing punch). In this case, the blank, in the region of the stiffening zone, is clamped in place in a positive-locking manner between fixing element and fixing punch.
In the course of the smoothing of the connection zone, the surface enlargement which has been produced during the vault structuring in the course of the production of the blank must be reduced in a specific manner. In order to avoid fold formation and/or the generation of radiating patterns on the structural part in the process, it is expedient to provide “consumers”, which ensure a defined flow of the excess material. Beads which are provided in the connection zone—preferably close to the transition to the stiffening zone—serve as such “consumers”. To shape these beads, bead regions are provided in the forming regions of the embossing tool.
The invention is explained in more detail below with reference to an exemplary embodiment shown in the drawings.